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+Strings
+=======
+
+We looked at strings, when looking at the sequence data-types of Python. We
+shall now look at them in a greater detail.
+
+So, what are strings? In Python anything within either single quotes or
+double quotes or triple single quotes or triple double quotes are strings.
+
+::
+
+  'This is a string'
+  "This is a string too'
+  '''This is a string as well'''
+  """This is also a string"""
+  'p'
+  ""
+
+Note that it really doesn't matter how many characters are present in the
+string. The last example is a null string or an empty string.
+
+Having more than one control character to define strings is handy when one of
+the control characters itself is part of the string. For example::
+
+  "Python's string manipulation functions are very useful"
+
+By having multiple control characters, we avoid the need for escaping
+characters -- in this case the apostrophe.
+
+The triple quoted strings let us define multi-line strings without using any
+escaping. Everything within the triple quotes is a single string no matter
+how many lines it extends
+
+::
+
+   """Having more than one control character to define
+   strings come as very handy when one of the control
+   characters itself is part of the string."""
+
+We can assign this string to any variable
+
+::
+
+  a = 'Hello, World!'
+
+Now ``a`` is a string variable. A string is a sequence of characters, as we
+have already seen. In addition string is an immutable collection. So all the
+operations that are applicable to any other immutable collection in Python
+works on string as well. So we can add two strings
+
+::
+
+  a = 'Hello'
+  b = 'World'
+  c = a + ', ' + b + '!'
+
+We can add string variables as well as the strings themselves all in the same
+statement. The addition operation performs the concatenation of two strings.
+
+Similarly we can multiply a string with an integer
+
+::
+
+  a = 'Hello'
+  a * 5
+
+gives another string in which the original string 'Hello' is repeated
+5 times.
+
+Let's now look at accessing individual elements of strings. Since, strings
+are collections we can access individual items in the string using the
+subscripts
+
+::
+
+  a[0]
+
+gives us the first character in the string. The indexing starts from 0
+for the first character and goes up to n-1 for the last character. We
+can access the strings from the end using negative indices
+
+::
+
+  a[-1]
+
+gives us the last element of the string and 
+
+::
+
+    a[-2]
+
+gives us second element from the end of the string
+
+Let us attempt to change one of the characters in a string::
+
+  a = 'hello'
+  a[0] = 'H'
+
+As said earlier, strings are immutable. We cannot manipulate a string.
+Although there are some methods which let us manipulate strings, we will look
+at them in the advanced session on strings. In addition to the methods that
+let us manipulate the strings we have methods like split which lets us break
+the string on the specified separator, the join method which lets us combine
+the list of strings into a single string based on the specified separator.
+
+Let us now learn to manipulate strings, specifically slicing and reversing
+them, or replacing characters, converting from upper to lower case and
+vice-versa and joining a list of strings.
+
+Let us consider a simple problem, and learn how to slice strings and get
+sub-strings.
+
+Let's say the variable ``week`` has the list of the names of the days of the
+week.
+
+::
+
+    week = ["sun", "mon", "tue", "wed", "thu", "fri", "sat"]
+
+
+Now given a string ``s``, we should be able to check if the string is a
+valid name of a day of the week or not. 
+
+::
+
+    s = "saturday"
+
+
+``s`` could be in any of the forms --- sat, saturday, Sat, Saturday,
+SAT, SATURDAY. For now, shall now be solving the problem only for the forms,
+sat and saturday. We shall solve it for the other forms, at the end of
+the tutorial. 
+
+So, we need to check if the first three characters of the given string
+exists in the variable ``week``. 
+
+As, with any of the sequence data-types, strings can be sliced into
+sub-strings. To get the first three characters of s, we say,
+
+::
+
+    s[0:3]
+
+Note that, we are slicing the string from the index 0 to index 3, 3
+not included. 
+
+::
+
+    s = "saturday"
+    s[:3]
+
+Now, we just check if that substring is present in the variable ``week``.
+
+::
+
+    s[:3] in week          
+
+Let us now consider the problem of finding out if a given string is
+palindromic or not. First of all, a palindromic string is a string that
+remains same even when it has been reversed.
+
+Let the string given be ``malayalam``.
+
+::
+
+    s = "malayalam"
+
+Now, we need to compare this string with it's reverse. 
+
+Again, we will use a technique common to all sequence data-types,
+[::-1]
+
+So, we obtain the reverse of s, by simply saying, 
+
+::
+
+    s[::-1]
+
+Now, to check if the string is ``s`` is palindromic, we say
+::
+
+    s == s[::-1]
+
+As, expected, we get ``True``. 
+
+Now, if the string we are given is ``Malayalam`` instead of ``malayalam``,
+the above comparison would return a False. So, we will have to convert the
+string to all lower case or all upper case, before comparing. Python provides
+methods, ``s.lower`` and ``s.upper`` to achieve this.
+
+Let's try it out. 
+::
+
+   s = "Malayalam"
+
+   s.upper()
+
+   s
+
+As you can see, s has not changed. It is because, ``upper`` returns a new
+string. It doesn't change the original string.
+
+::
+
+   s.lower()
+
+   s.lower() == s.lower()[::-1]
+   
+So, as you can see, now we can check for presence of ``s`` in ``week``, in
+whichever format it is present -- capitalized, or all caps, full name or
+short form.
+
+We just convert any input string to lower case and then check if it is
+present in the list ``week``.
+
+Now, let us consider another problem. We often encounter e-mail id's which
+have @ and periods replaced with text, something like info[at]fossee[dot]in.
+We now wish to get back proper e-mail addresses.
+
+Let's say the variable email has the email address.
+
+::
+
+   email = "info[at]fossee[dot]in"
+
+Now, we first replace the ``[at]`` with the ``@``, using the replace method
+of strings.
+
+::
+
+   email = email.replace("[at]", "@")
+   print email
+
+   email = email.replace("[dot]", ".")        
+   print email
+
+Now, let's look at another interesting problem where we have a list of e-mail
+addresses and we wish to obtain one long string of e-mail addresses separated
+by commas or semi-colons.
+
+::
+
+  email_list = ["info@fossee.in", "enquiries@fossee.in",  "help@fossee.in"]
+
+Now, if we wish to obtain one long string, separating each of the
+email id by a comma, we use the join operator on ``,``. 
+
+::
+
+  email_str = ", ".join(email_list)
+  print email_str
+
+Notice that the email ids are joined by a comma followed by a space. 
+
+That brings us to the end of our discussion on strings. Let us now look at
+conditionals. 
+
+Conditionals
+============
+
+Whenever we have two possible states that can occur depending on a whether a
+certain condition we can use if/else construct in Python.
+
+For example, say, we have a variable ``a`` which stores integers and we are
+required to find out whether ``a`` is even or odd. an even number or an odd
+number. Let's say the value of ``a`` is 5, now. 
+
+::
+
+  a = 5
+
+In such a case we can write the if/else block as
+
+::
+
+  if a % 2 == 0:
+      print "Even"
+  else:
+      print "Odd"
+
+If ``a`` is divisible by 2, i.e., the result of "a modulo 2" is 0, it prints
+"Even", otherwise it prints "Odd".
+
+Note that in such a case, only one of the two blocks gets executed depending
+on whether the condition is ``True`` or ``False``.
+
+There is a very important sytactic element to understand here. Every code
+block begins with a line that ends with a ``:``, in this example the ``if``
+and the ``else`` lines. Also, all the statements inside a code block are
+intended by 4 spaces. Returning to the previous indentation level, ends the
+code block.
+
+The if/else blocks work for a condition, which can take one of two states.
+What do we do for conditions, which can take more than two states?
+
+Python provides if/elif/else blocks, for such conditions. Let us take an
+example. We have a variable ``a`` which holds integer values. We need to
+print "positive" if ``a`` is positive, "negative" if it is negative or "zero"
+if it is 0.
+
+Let us use if/elif/else ladder for it. For the purposes of testing our
+code let us assume that the value of a is -3
+
+::
+
+  a = -3
+
+  if a > 0:
+      print "positive"
+  elif a < 0:
+      print "negative"
+  else:
+      print "zero"    
+
+All the syntax and rules as said for if/else statements hold. The only
+addition here is the ``elif`` statement which can have another condition of
+its own.
+
+Here too, exactly one block of code is executed -- the block of code which
+first evaluates to ``True``. Even if there is a situation where multiple
+conditions evaluate to True all the subsequent conditions other than the
+first one which evaluates to True are neglected. Consequently, the else block
+gets executed if and only if all the conditions evaluate to False.
+
+Also, the ``else`` block in both if/else statement and if/elif/else is
+optional. We can have a single if statement or just if/elif statements
+without having else block at all. Also, there can be any number of elif's
+within an if/elif/else ladder. For example
+
+::
+
+  if user == 'admin':
+      # Do admin operations
+  elif user == 'moderator':
+      # Do moderator operations
+  elif user == 'client':
+      # Do customer operations
+
+is completely valid. Note that there are multiple elif blocks and there
+is no else block.
+
+In addition to these conditional statements, Python provides a very
+convenient ternary conditional operator. Let us take the following example
+where we have a score string, which can either be a number in the range 0 to
+100 or the string 'AA', if the student is absent. We wish to convert the
+score string, into an integer, whenever possible. If the score string is
+'AA', we wish to make the corresponding value 0. Let us say the string score
+is stored in score_str variable. We can do it using an ``if-else`` construct
+as below
+
+::
+
+    if score_str != 'AA':
+        score = int(score_str)
+    else:
+        score = 0
+
+The same thing can be done using a ternary operator, which reads more natural
+and has greater brevity. 
+
+::
+
+    score = int(score_str) if score_str != 'AA' else 0
+
+Moving on, there are certain situations where we will have no operations or
+statements within a block of code. For example, we have a code where we are
+waiting for the keyboard input. If the user enters "c", "d" or "x" as the
+input we would perform some operation nothing otherwise. In such cases "pass"
+statement comes very handy.
+
+::
+
+  a = raw_input("Enter 'c' to calculate and exit, 'd' to display the existing
+  results exit and 'x' to exit and any other key to continue: ")
+
+  if a == 'c':
+     # Calculate the marks and exit
+  elif a == 'd':
+     # Display the results and exit
+  elif a == 'x':
+     # Exit the program
+  else:
+     pass
+
+In this case "pass" statement acts as a place holder for the block of code.
+It is equivalent to a null operation. It literally does nothing. It can used
+as a place holder when the actual code implementation for a particular block
+of code is not known yet but has to be filled up later.
+
+That brings us to the end of our discussion of conditionals. 
+
+Loops
+=====
+
+We shall now, look at ``while`` and ``for`` loops. We shall look at how to
+use them, how to break out of them, or skip some iterations in loops.
+
+We shall first begin with the ``while`` loop. The ``while`` loop is used for
+repeated execution as long as a condition is ``True``.
+
+Let us print the squares of all the odd numbers less than 10, using the
+``while`` loop.
+
+::
+
+    i = 1
+
+    while i<10:
+        print i*i
+        i += 2
+
+This loop prints the squares of the odd numbers below 10. 
+
+The ``while`` loop, repeatedly checks if the condition is true and executes
+the block of code within the loop, if it is. As with any other block in
+Python, the code within the ``while`` block is indented to the right by 4
+spaces.
+
+Let us now solve the same problem of printing the squares of all odd numbers
+less than 10, using the ``for`` loop. The ``for`` loop iterates over a list
+or any other sequential data type. 
+
+::
+
+    for n in [1, 2, 3]:
+        print n
+
+Each of the elements of the list, gets printed. The variable ``n``, called
+the loop variable, successively takes the value of each of the elements in
+the list, in each iteration. 
+
+Now, we could solve the problem of calculating the squares, by 
+
+::
+
+    for n in [1, 3, 5, 7, 9]: 
+        print n*n
+
+But, it is "unfair" to generate the list by hand. So, we use the ``range``
+function to get a list of odd numbers below 10, and then iterate over it and
+print the required stuff.
+
+::
+
+    for n in range(1, 10, 2):
+        print n*n
+
+The first argument to the ``range`` function is the start value, the second
+is the stop value and the third is the step-size. The ``range`` function
+returns a list of values from the start value to the stop value (not
+included), moving in steps of size given by the step-size argument. 
+
+Also, The start and the step values are optional. For instance, the code
+below prints numbers from 0 to 9. 
+
+::
+
+    for n in range(10):
+        print n
+
+Let us now look at how to use the keywords, ``pass``, ``break`` and
+``continue``.
+
+As we already know, ``pass`` is just a syntactic filler. It is used
+for the sake of completion of blocks, that do not have any code within
+them. 
+
+::
+
+    for n in range(2, 10, 2):
+        pass
+
+``break`` is used to break out of the innermost loop. The ``while``
+loop to print the squares of all the odd numbers below 10, can be
+modified using the ``break`` statement, as follows
+::
+
+    i = 1
+
+    while True:
+        print i*i
+        i += 2
+        if i<10:
+            break
+
+``continue`` is used to skip execution of the rest of the loop on this
+iteration and continue to the end of this iteration. 
+
+Say, we wish to print the squares of all the odd numbers below 10, which are
+not multiples of 3, we would modify the ``for`` loop as follows. 
+
+::
+
+    for n in range(1, 10, 2):
+        if n%3 == 0:
+            continue      
+        print n*n
+  
+This brings us to the end of the section on loops. We have learned how to use
+the ``for`` and ``while`` loops. 
+
+Lists
+=====
+
+We have already seen lists as a kind of sequence data-type. We shall look at
+them in greater detail, now. 
+
+We will first create an empty list with no elements. 
+
+::
+
+    empty = [] 
+    type(empty)
+   
+This is an empty list without any elements.
+
+Let's now define a non-empty list. 
+
+::
+
+     p = ['spam', 'eggs', 100, 1.234]
+
+Thus the simplest way of creating a list is typing out a sequence of
+comma-separated values (or items) between two square brackets.
+
+As we can see lists can contain different kinds of data. They can be
+heterogeneous. In the previous example 'spam' and 'eggs' are strings whereas
+100 and 1.234 are integer and float respectively. Below, is another example. 
+
+::
+
+      q = [[4, 2, 3, 4], 'and', 1, 2, 3, 4]
+
+As you already know, we access an element of a list using its index. Index of
+the first element of a list is 0. 
+
+::
+    p[0] 
+    p[1] 
+    p[3]
+
+
+List elements can also be accessed using negative indexing. p[-1]
+gives the last element of p. 
+
+::
+    p[-1]
+
+As you can see you get the last element which is 1.234.
+
+Similarly, -2 gives the second to last element and -4 gives the fourth from
+the last which, in this case, is the first element.
+
+::
+   
+    p[-2] 
+    p[-4]
+
+Using ``len`` function we can check the number of elements in the list
+p. 
+
+::
+	 
+    len(p)
+
+We can append elements to the end of a list using the method append.
+
+::
+
+    p.append('onemore') 
+    p
+    p.append([1, 6])
+    p
+   
+As we can see ``p`` is appended with 'onemore' and [1, 6] at the end.
+
+Just like we can append elements to a list we can also remove them. There are
+two ways of doing it. First, is by using the ``del`` command and the index of
+the element. 
+
+::
+
+    del p[1]
+
+
+
+will delete the element at index 1, i.e the second element of the list,
+'eggs'. 
+
+The other way is removing element by choosing the item. Let's say one wishes
+to delete 100 from p list the syntax of the command would be
+
+::
+
+    p.remove(100)
+
+but what if there were two 100's. To check that lets do a small
+experiment. 
+
+::
+
+    p.append('spam') 
+    p
+    p.remove('spam') 
+    p
+
+If we check now we will see that the first occurence 'spam' is removed and
+therefore `remove` removes the first occurence of the element in the sequence
+and leaves others untouched.
+
+Another other basic operation that we can perform on lists is concatenation
+of two or more lists. We can combine two lists by using the "plus" operator.
+Say we have
+
+::
+
+    a = [1, 2, 3, 4]
+    b = [4, 5, 6, 7]
+    a + b
+
+When we concatenate lists using the "plus" operator we get a new list. We can
+store this list in a new variable
+
+::
+
+    c = a + b
+    c
+
+It is important to observe that the "plus" operator always returns a new list
+without altering the lists being concatenated in any way.
+
+Let us now look at slicing and striding on lists. Let's create a list primes.
+
+::
+
+    primes = [2, 3, 5, 7, 11, 13, 17, 19, 23, 29]
+
+To obtain all the primes between 10 and 20 from the above list of primes we
+say
+
+::
+
+    primes[4:8]
+
+This gives us all the elements in the list starting from the element with the
+index 4, which is 11 in our list, upto the element with index 8 (not
+included). 
+
+::
+
+    primes[0:4]
+
+will give us the primes below 10. Recall that the element with the index 4 is
+not included in the slice that we get. 
+
+By default the slice fetches all the elements between start and stop (stop
+not-included). But, Python also provides the functionality to specify a step
+size, when picking elements. Say, we have
+
+::
+
+    num = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13]
+
+If we want to obtain all the odd numbers less than 10 from the list
+``num`` we have to start from element with index 1 upto the index 10 in
+steps of 2
+
+::
+
+    num[1:10:2]
+
+When no step is specified, it is assumed to be 1. Similarly, there are
+default values for start and stop indices as well. If we don't specify the
+start index it is implicitly taken as the first element of the list
+
+::
+
+    num[:10]
+
+This gives us all the elements from the beginning upto the 10th element but
+not including the 10th element in the list "num". Similary if the stop index
+is not specified it is implicitly assumed to be the end of the list,
+including the last element of the list
+
+::
+
+    num[10:]
+
+gives all the elements starting from the 10th element in the list
+"num" upto the final element including that last element. Now
+
+::
+
+    num[::2]
+
+gives us all the even numbers in the list "num".
+
+We know that a list is a collection of data. Whenever we have a collection we
+run into situations where we want to sort the collection. Lists support sort
+method which sorts the list in-place
+
+::
+
+    a = [5, 1, 6, 7, 7, 10]
+    a.sort()
+
+Now the contents of the list ``a`` will be
+
+::
+
+    a
+    [1, 5, 6, 7, 7, 10]
+
+As the sort method sorts the elements of a list, the original list we had is
+overwritten or replaced. We have no way to obtain the original list back. One
+way to avoid this is to keep a copy of the original list in another variable
+and run the sort method on the list. 
+
+However, Python also provides a built-in function called sorted which sorts
+the list which is passed as an argument to it and returns a new sorted list
+
+::
+
+    a = [5, 1, 6, 7, 7, 10]
+    sorted(a)
+  
+We can store this sorted list another list variable
+
+::
+
+    sa = sorted(a)
+
+Python also provides the reverse method which reverses the list in-place
+
+::
+
+    a = [1, 2, 3, 4, 5]
+    a.reverse()
+
+reverses the list ``a`` and stores the reversed list inplace i.e. in ``a``
+itself. Let's see the list ``a``
+
+::
+
+    a
+    [5, 4, 3, 2, 1]
+
+But again the original list is lost. 
+
+To reverse a list, we could use striding with negative indexing.
+
+::
+
+    a[::-1]
+
+We can also store this new reversed list in another list variable.
+
+That brings us to the end of our discussion on lists. 
+
+.. 
+   Local Variables:
+   mode: rst
+   indent-tabs-mode: nil
+   sentence-end-double-space: nil
+   fill-column: 77
+   End:
+
+